The following heat maps display two variables, the median age (years) and the unemployment rate (%), of countries by using colors on the world map (data source: CIA's World Factbook).
It depends: If your target is to visualize every single variable clearly, the concept works. If your target is to help your audience understand the relationship between variables, then an XY chart works better (such as Google's motion chart--here without the animation):
Let's analyze: By average, the lower the median age is in a country, the higher the unemployment rate.
UPDATE (Thursday, April 17, 2008):
Included below a more traditional chart to display the relationship between median age and unemployment rate. In this chart, countries are grouped in median age groups. An average unemployment rate is calculated for each group.